With existing tire polymer and compound technology there is a tradeoff between desirable tire properties using commonly used tire polymers. The impending introduction of fire labeling regulations, along with increased competition make it more important than ever to produce tires that show high performance in every category. Tire properties are directly related to the material properties of the polymers used in the tire, which are in turn directly related to the glass transition temperature (Tg) of the chosen polymer. The most commonly used tire polymers are cis-polybutadiene, natural rubber, high-vinyl polybutadiene, and styrene/butadiene copolymers; these polymers are largely chosen for different roles in tire compounds based on their Tg. The traditional relationships between polymer Tg and three important tire performance properties are outlined in the following table.
Performance Effect of characteristicincreasing TgRolling resistanceworsenedTread wearworsenedWet tractionimproved
This relationship between Tg, tan delta, and tire properties can also be visualized by examining the tan delta curves of various polymers. The value of tan delta at various temperatures is commonly used as an indicator of polymer performance, for example, the tan delta at 0° C. is an indicator of wet performance, while the tan delta at 60° C. is an indicator of rolling resistance. A tire with a higher tan delta at 0° C., such as an SBR, also exhibits a high tan delta at 60° C., making it a good choice for wet performance, but bad for rolling resistance. The opposite is true for cis-polybutadiene: the tan delta is low at both 0° C. and 60° C., making cis-PBD an excellent choice for improved rolling resistance but poor for wet traction performance.
There is a tradeoff between wet performance and the other two important characteristics, and it would seem at first glance that there is no way around this natural tradeoff. However, the shape of the tan delta/temperature curve also influences the polymer properties. Polymers of different composition exhibit different relationships between their glass transition temperatures and physical properties. An ideal polymer would exhibit a higher Tg and a steeper tan delta/temperature slope, allowing tan delta to be as high as possible in the wet traction regime and as low as possible in the rolling resistance regime. This ideal polymer is not known among existing tire elastomers, therefore there is a need for new technology to prepare new classes of polymers with properties approaching those of this ideal polymer.
Both high-cis polybutadiene rubber and low-cis butadiene/styrene copolymers have achieved widespread use as tire polymers, and are commonly used in blends in tire tread compounds to balance RR and traction properties. To date, however, there is no example of a polymer that combines the major aspects of these two important polymers, that is to say there are no established butadiene/styrene copolymers that contain a high-cis butadiene backbone.